The implementation of the Wendelstein 7-X control a data acquisition concepts at VUV/XUV overview spectrometers HEXOS

HEXOS (high efficiency extreme ultraviolet overview spectrometer) is an optimized set of four efficient VUV/XUV spectrometers. It is suitable for a complete coverage of the wavelength range of interest with sufficient spectral resolution. The spectrometers cover the entire wavelength range of 2.5–160 nm with high performance (up to 9999 spectra at spectra rate of 1000 s^?1). To operate according to the Wendelstein 7-X (W7-X) control and data acquisition guidelines all necessary concepts for safety, autonomous and subordinated operation, and segment program controlled experiment operation will be implemented at HEXOS. The design of the HEXOS control and data acquisition system and the implementation of the main W7-X control and data acquisition concepts are described. An outlook on the test phase at the TEXTOR (Tokamak Experiment for Technology Oriented Research) device and the commissioning phase at W7-X is given.     

Gate valve and shutter control system of the fusion experiment Wendelstein 7-X

The plasma vessel of the fusion experiment Wendelstein 7-X (W7-X) is a plasma vessel covering a plasma volume of about 30 m³. The vacuum conditions for plasma experiments inside the plasma vessel are supposed to be in a range of 1 × 10⁻⁸ mbar (ultra high vacuum conditions) after evacuation and conditioning. The 254 ports of the plasma vessel allow an external access to the inner space of the plasma vessel. Ports for heating and diagnostic systems are equipped with gate valves or with shutters. The vacuum gate valves are used as a controllable mechanical and a vacuum disconnection point between diagnostics and heating systems on the port side and the inner plasma vessel on the other side. The shutters are responsible for an optical and thermal protection for port windows or installed equipments inside the ports. After an overview of the main requirements for the control of the huge number of gate valves and shutters for the operational phases 1 and 2 of W7-X the design and realization of a centralized control system for controlling and observing all shutters and the majority of gate valves of the machine Wendelstein 7-X will be introduced and discussed.     

Manufacturing of the Wendelstein 7-X divertor and wall protection

The in-vessel components of Wendelstein 7-X (W7-X) with a total surface of 265 m² comprise the divertor and the wall protection. The high heat flux (HHF) and lower heat flux (LHF) target, the baffle, the end plates closing the divertor chamber, a cryo vacuum pump (CVP) and a control coil form one divertor unit. Steel panels and the graphite heat shield protect the wall, including the ports. The HHF target elements, the steel panels and the control coils are manufactured by industry. The remaining components will be manufactured by the Max-Planck-Institute für Plasmaphysik (IPP) at its Garching workshops. For all components the final acceptance tests will be performed by IPP. This paper summarizes the main aspects for manufacturing, the preceding development and qualification tests as well as the final acceptance tests for the in-vessel components.     

Overview of main-mechanical-components and critical manufacturing aspects of the Wendelstein 7-X cryostat

Wendelstein 7-X (W7-X) will be the world's largest superconducting helical advanced stellarator. This stellarator concept is deemed to be a desirable alternative for a future power plant like DEMO. The main advance of the static plasma is caused by the three dimensional shape of some of the main mechanical component inside the cryostat. The geometry of the plasma vessel is formed around the three dimensional shape of the plasma. The coils and their support structure are enclosed within the outer vessel. The space between the outer, the plasma vessel and the ports is called cryostat because the vacuum inside provides thermal insulation of the magnet system which is cooled down to 4 K. Due to the different thermal movements of both vessels and the support structure have to be supported separately. 10 cryo legs will bear the coil support structure. The plasma vessel supporting system is divided into two separate systems, allowing horizontal and vertical adjustments. This paper aims to give an overview of the main mechanical components of the cryostat. The authors delineate some disparate and special problems during the manufacturing of the components at the companies in Europe. It describes the current manufacturing and assembly.     

Manufacturing and assembly status of main components of the Wendelstein 7-X cryostat

The stellarator fusion experiment Wendelstein 7-X (W7-X) is at present in assembly at the Max-Planck Institut für Plasmaphysik (IPP).The toroidal plasma with a ring diameter of 11 m and an average plasma diameter of 1.1 m is contained within the plasma vessel. Its form is dictated by the shape of the plasma. The form of the plasma is controlled by the coil system configuration. To control the plasma form it is necessary that all the 20 planar and 50 non-planar coils should be positioned within a tolerance of 1.5 mm. To meet this requirement a complex coil support structure was created, consisting of the central support ring and the different inter coil supports. The coils and the support structure are enclosed within the outer vessel with its domes and openings. The space between the outer and the plasma vessel is called cryostat because the vacuum inside provides thermal insulation of the magnet system, and the entire magnetic system is then be cooled down to 4 K. Due to the different thermal movements the plasma vessel and the central support ring have to be supported separately. The central support ring is held by 10 cryo legs. The plasma vessel supporting system is divided into two separate systems, allowing horizontal and vertical adjustments to centre the plasma vessel during thermal expansion.This paper aims to give an overview of the main components in the cryostat like the plasma vessel, the outer vessel, the ports and the different support systems. It describes the current manufacturing and assembly status and the associated problems of these components, using pictures and text. This paper does not describe the general assembly situation or time schedules of the Wendelstein 7-X.     

Design and manufacturing status of trim coils for the Wendelstein 7-X stellarator experiment

The stellarator fusion experiment Wendelstein 7-X (W7-X) is currently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany. The main magnetic field will be provided by a superconducting magnet system which generates a fivefold toroidal periodic magnetic field. However, unavoidable tolerances can result in small deviations of the magnetic field which disturb the toroidal periodicity. In order to have a tool to influence these field errors five additional normal conducting trim coils were designed to allow fine tuning of the main magnetic field during plasma operation. In the frame of an international cooperation the trim coils will be contributed by the US partners. Princeton Plasma Physics Laboratory has accomplished several tasks to develop the final design ready for manufacturing e.g. detailed manufacturing design for the winding and for the coil connection area. The design work was accompanied by a detailed analysis of resulting forces and moments to prove the design. The manufacturing of the coils is running at Everson Tesla Inc; the first two coils were received at IPP.     

Design,manufacture and testing of the glow discharge electrodes for Wendelstein 7-X

The electrodes for the Wendelstein 7-X glow discharge system have been designed, tested and manufactured. The compact design relies on a cooled housing, integrated into the first wall cooling system, and a calotte-shaped graphite anode. The new mounting concept avoids the need of active cooling of the anode due to an improved thermal conduction. Comprehensive tests of a prototype electrode had been carried out in laboratory and in the ASDEX Upgrade Tokamak during two operation campaigns. The electrode showed excellent and reliable long-time discharge behavior and fulfilled all the requirements regarding temperature limits and maintainability resulting from the steady-state operation of W7-X.     

Manufacturing and assembly of the plasma- and outer vessel of the cryostat for Wendelstein 7-X

Wendelstein 7-X is an advanced helical stellarator, which is presently under construction at the Greifswald branch of IPP. A set of 70 superconducting coils arranged in five modules provides a twisted shaped magnetic cage for the plasma and allows steady state operation. Operation of the magnet system at cryogenic temperatures requires a cryostat which provides thermal protection and gives access to the plasma. The main components of the cryostat are the plasma vessel, the outer vessel, the ports, and the thermal insulation. The German company, MAN Diesel & Turbo SE Deggendorf (former MAN DWE GmbH Deggendorf), is responsible for the manufacture and assembly of the plasma vessel, the outer vessel and the thermal insulation. This paper describes the manufacturing and assembly technology of the plasma and outer vessel of the cryostat for Wendelstein 7-X.     

The procurement and testing of the stainless steel in-vessel panels of the Wendelstein 7-X Stellarator

320 In-vessel water cooled stainless steel panels, poloidal closure plates and pumping gap panels, covering an area of approximately 100 m², are used in Wendelstein7-X to protect the plasma vessel. The panels are manufactured at Deggendorf, Germany by MAN Diesel & Turbo SE. The panels consist of a laser welded sandwich of stainless steel plates together with a labyrinth of cooling channels and have a complicated geometry to fit the plasma vessel of Wendelstein 7-X. The hydraulic and mechanical stability requirements whilst maintaining the tight tolerances for the shape of the components are very demanding. The panels are designed to operate at up to an average heat load of 100 kW/m² and a maximum heat load of 200 kW/m² with a water velocity of approximately 2 m s^?1. High heat flux testing of an un-cooled panel at a time averaged load of 200 kW/m² for 10 s were successfully performed to support the start up phase of Wendelstein 7-X operation. Extensive testing both during manufacture and after delivery to IPP-Garching demonstrates the suitability of the delivered panels for their purpose.     

Status of series production and test of the HTS current leads for Wendelstein 7-X

The Karlsruhe Institute of Technology (KIT) is responsible for design, production and test of the High Temperature Superconductor (HTS) current leads for the stellarator Wendelstein 7-X (W7-X). In total 14 current leads with a maximum current of 18.2 kA are required. Special feature is the upside-down orientation of the current leads because of the location of the power supplies in the basement of the experimental area of W7-X. One further important requirement is the Paschen tight electrical insulation of current leads and the connection to the bus bar system. Due to some very specific manufacturing steps, budget and time restrictions, it has been mutually decided between the project partners to manufacture most of the components in house, except the HTS stacks which have been produced and delivered by industry. As the semi-finished parts were manufactured in the central workshop of KIT, the assembly of the current leads was performed in the ITEP (Institute for Technical Physics). The final acceptance test of the current leads is performed at KIT, using a dedicated test cryostat assembled beside and connected to the main vacuum vessel of the TOSKA facility. The paper describes the status of the manufacturing of the current leads. In addition attention is given to specific problems that occurred during the manufacturing and testing.     

Lessons learned from the design and fabrication of the baffles and heat shields of Wendelstein 7-X

The baffles and heat shields of the wall protection of the Wendelstein 7-X stellarator are actively water cooled components based on the same technology. Fine grain graphite tiles are clamped onto a CuCrZr heat sink, which is vacuum brazed to a stainless steel tube. The baffles are part of the divertor and improve the divertor pumping efficiency. The heat shields protect the plasma vessel wall, water piping, cables and the integrated diagnostics. The 170 baffles with 25 variants and 162 heat shield modules with 85 variants comprise a total surface of 33 m² and 51 m², respectively. Design guidelines enabled as much as possible the standardization of the fabrication to allow for a more efficient work organization. Individual jigs have been manufactured for each variant in order to weld, bend and mill the different parts of the baffles and heat shields to the required 3D accuracy. At the end of the manufacturing process, each component has been checked and documented according to a detailed quality plan.     

Spectroscopic studies of fuel recycling and impurity behaviors in the divertor region of Wendelstein 7-X

The first divertor operation phase(OP1.2 a) was carried out on Wendelstein 7-X in the second half of 2017.Fuel recycling and impurity behaviors in the divertor region were investigated by employing a newly built ultraviolet–visible–near infrared overview spectroscopy system.The characteristic spectral lines of the working gases(hydrogen and helium),intrinsic impurities(carbon,oxygen and iron),and seeded impurities(neon and nitrogen) were identified and analyzed.The divertor electron temperature and density were measured using He I(667.8,706.5,and 728.1 nm) line intensity ratios.The Hα(656.3 nm),He I(587.6 nm),C II(514.5 nm),and O I(777.2 nm) emissions were investigated over a wide range of operating conditions.The results showed that fuel and impurity emissions in the divertor region exhibit a strong dependence on magnetic topology and plasma conditions.The levels of Hα,He I,C II,and O I emissions are all reduced moving from the standard configuration to the high mirror configuration,and even further reduced for the high iota configuration,which is associated with decreasing connection length in these island divertor configurations.The H/He influx ratio shows that the plasma is a mixture of helium and hydrogen.The neutral and impurity influxes from the divertor target tend to increase with increasing divertor electron temperature.     

Mechanical analysis of the joint between Wendelstein 7-X target elements and the divertor frame structure

The target elements of the actively cooled high heat flux (HHF) divertor of Wendelstein 7-X are made of CFC (carbon fiber-reinforced carbon composite) tiles bonded to a CuCrZr heat sink and are mounted onto a support frame. During operation, the power loading will result in the thermal expansion of the target elements. Their attachment to the support frame needs to provide, on the one hand, enough flexibility to allow some movement to release the induced thermal stresses and, on the other hand, to provide enough stiffness to avoid a misalignment of one target element relative to the others. This flexibility is realized by a spring element made of a stack of disc springs together with a sliding support at one of the two or three mounting points. Detailed finite element calculations have shown that the deformation of the heat sink leads to some non-axial deformation of the spring elements. A mechanical test was performed to validate the attachment design under cyclic loading and to measure the deformations typical of the expected deformation of the elements. The outcome of this study is the validation of the design selected for the attachment of the target elements, which survived experimentally the applied mechanical cycling which simulates the thermal cycling under operation.     

Development of a thermo-hydraulic bypass leakage test method for the Wendelstein 7-X target element cooling structure

A facility for testing the cooling structure to ensure the quality of the target element heat sink is under construction at IPP Garching. This test bed has been built up to do a proof of concept study with a hydraulic mock up, built from the same material as the target elements, but can be opened for artificial gap manufacture.A bypass in the cooling structure was the main reason for the overheating of CFC tiles, which resulted in a defect by cracking of the CFC–AMC-Cu interlayer in pre-series 3. The cooling structure is built from two half-shells, in both a half pipe is milled [1]. To correlate the thermal response function to the artificial bypass gaps the time constant of the cool down was used. The results of the measurements are presented and compared with the calculated results.     

Towards assembly completion and preparation of experimental campaigns of Wendelstein 7-X in the perspective of a path to a stellarator fusion power plant

The superconducting stellarator device Wendelstein 7-X, currently under construction, is the key device for the proof of stellarator optimization principles. To establish the optimized stellarator as a serious candidate for a fusion reactor, reactor-relevant dimensionless plasma parameters must be achieved in fully integrated steady-state scenarios. After more than 10 years of construction time, the completion of the device is now approaching rapidly (mid-2014). We discuss the most important lessons learned during the device assembly and first experiences with coming major work packages. Those are (a) assembly of about 2500 large, water-cooled, 3d-shaped in-vessel component elements; (b) assembly of in total 14 superconducting current leads, one pair for each coil type; and (c) assembly of the device periphery including diagnostics and heating systems. In the second part we report on the present status of planning for the first operation phase (5–10 s discharge duration at 8 MW heating power), the completion and hardening of the device for full power steady-state operation, and the second operation phase (up to 30 min discharge duration at 10 MW heating power). It is the ultimate goal of operation phase one to develop credible and robust discharge scenarios for the high-power steady-state operation phase two. Beyond the improved equilibrium, confinement, and stability properties owing to stellarator optimization, this requires density control, impurity control, edge iota control as well as high density microwave heating. Of paramount importance is the operation of the island divertor, which is realized in the first operation phase as an inertially cooled conventional graphite target divertor. It will be replaced later on by the steady-state capable island divertor with its water-cooled carbon fiber reinforced carbon target elements.     

Production management and quality assurance for the fabrication of the In-Vessel Components of the stellarator Wendelstein 7-X

The In-Vessel Components (IVC) of the stellarator Wendelstein 7-X consist of the divertor components and the first wall (FW) with their internal water cooling supply and a set of diagnostics. Due to the significant amount of different components, including many variants, a tool called Production Managing System (PMS) has been developed to organize the fabrication and the associated quality assurance. The PMS works by building a database containing the basic parts and assembly data, manufacturing and quality control plans, and available machine capacity. The creation of this database is based mainly on the parts lists, the manufacturing drawings, and details of the working flow organization. As a consequence of the learning process and technical adjustments during the design and manufacturing phase, the database needed to be permanently updated. Therefore an interface tool to optimize the data preparation has been developed. PMS has been demonstrated to be an efficient tool to support the IVC production activities providing reliable planning estimates, easily adaptable to problems encountered during the fabrication and provided a basis for the integration of quality assurance requirements.     

Results and consequences of high heat flux testing as quality assessment of the Wendelstein 7-X divertor

The series manufacturing of the first 282 Wendelstein 7-X divertor elements was concluded in 2011. The divertor is designed to remove a steady-state heat load of 10 MW/m². 940 target elements of five different types made of CuCrZr heat sinks and covered with 16,000 CFC NB31 flat-tiles have to be produced. Additional to quality assessment during the manufacturing process, a final assessment of the delivered elements with operational heat load is indispensable to ensure a constant high thermal performance of the installed divertor.Based on the results of the pre-series testing a statistical quality assessment method has been developed for the series production. The application of this method to the series elements ensures their thermal performance with reasonable high heat flux test effort.     

Performance and statistical quality assessment of CFC tile bonding on the pre-series elements of the Wendelstein 7-X divertor

Extensive high heat flux (HHF) testing of pre-series IV targets was performed to establish the industrial process for the ongoing production of the actively water-cooled target elements which will be needed for the installation of the Wendelstein 7-X (W7-X) divertor. Finally, 890 components covered with about 18,000 CFC tiles will be installed.The examinations of the elements with 10 MW/m² cycling up to 10,000 pulses, 16 MW/m² cycling and screening tests up to 32 MW/m², confirm the robustness of the design and in particular of the applied CFC bonding technology. The results of the IR examination of the initial tests have been assessed statistically. The paper presents a detailed statistical analysis based on the Six-Sigma method of the surface temperature increase of the CFC tiles tested for 100 cycles at 10 MW/m². Assuming that the series elements will behave in a similar fashion to the pre-series elements this statistical assessment can also be performed for the series elements.     

New developments at JET in diagnostics, real-time control, data acquisition and information retrieval with potential application to ITER

In magnetic confinement fusion, the operation of next generation devices will be significantly different compared to present day machines. The duration length of the discharges will require abandoning the traditional paradigm of processing and storing the data after the shot. In fact most information will have to be made available in real-time. The significant issues of machine protection will require more sophisticated and at the same time more robust feedback control schemes. Another very important issue emerged in the last years of JET operation, and which is expected to become more severe in ITER, is the large amount of data to be analysed, which cannot be handled in the most efficient way with traditional methods.In order to prepare for the operation of ITER, some tests are being performed at JET. The capacity of the real-time network has increased in the last years, and many more systems, mainly diagnostics have been connected to it in order to test their reliability and to assess the quality of the information they can provide for feedback control. To reduce the amount of data, a prototype of real-time adaptive data acquisition techniques is being implemented, to adjust the acquisition frequency to the time resolution of the phenomena to be analysed in the plasma. Lossless data compression techniques have been refined and various intelligent signal processing methods have already been implemented to allow an easier and more objective first screening of the data. To allow scientists from wide and diffuse communities to participate in the scientific and technical programme, various innovative tools for remote participation and experimentation are also being actively investigated.